The present invention relates to methods and apparatus for reflecting and focusing electromagnetic radiation and, more particularly, to a low-cost, tunable, steerable, reconfigurable reflector array which simulates the electromagnetic effects of a parabolic or other non-planar reflector by means of a flat surface.
In the field of radio frequency (RF) communications, it is often necessary and/or desirable to be able to focus, direct, or otherwise manipulate an RF signal. Traditionally this has been accomplished by placing a reflective surface in the signal path, either to gather and focus a weak signal being received or to concentrate a transmitted signal. While flat surfaces reflect RF energy, their effect is very much like an optical mirror in that they reflect an incident signal at an orthogonal angle to the angle of incidence and, consequently, perform no concentrating or focusing function. The use of a curved (e.g., a parabolic) surface, however, does provide a concentrating, focusing function.
One problem with a curved physical structure is that space must be provided to accommodate its depth. This is not always easy, especially on aircraft or spacecraft where a smooth outer surface skin may be essential. Because the operation of a parabolic or similar reflective surface is well understood, attempts have been made to simulate the function of such surfaces without the need for a physically deep structure.
U.S. Pat. No. 4,905,014 for MICROWAVE PHASING STRUCTURES FOR ELECTROMAGNETICALLY EMULATING REFLECTIVE SURFACES AND FOCUSING ELEMENTS OF SELECTED GEOMETRY, issued to Daniel G. Gonzalez, et al., provides one such solution. GONZALEZ, et al. teach a planar surface consisting of an array of elements, each functioning as both a radiator and a phase shifter. Crossed, shorted dipoles are positioned approximately xe2x85x9 xcex above a ground plane for impedance matching purposes. This distance of xe2x85x9 xcex is chosen because that is the point where signals reflected from the bottom ground plane are 90xc2x0 out of phase (i.e., in quadrature). When considering phase shifts, 90xc2x0 is the maximum phase shift that can occur. The tuned dipoles may be arranged to match this phase or to algebraically add to it. At this distance, neither the E nor H fields are peaking. In addition, the selected distance ensures that the signal is not out of phase with the ground plane.
Incident RF energy causes a standing wave to be set up between the elements and the ground plane. Each dipole element has an RF reactance near its resonant frequency, which, combined with the standing wave, causes radiant RF to be re-radiated with a known phase shift, controllable by the dipole""s length and other specific physical parameters such as the dielectric constant of the material upon which the dipoles are supported, etc.
However, there are shortcomings of this approach. The operational parameters (i.e., operating frequency and directional characteristics) of the array are fixed. The GONZALEZ, et al. array, for example, is operable over only a relatively narrow frequency band and its directionality is fixed. The inventive array, on the other hand, utilizes microelectromechanical systems (MEM) switches or the like to control the length of the dipole elements to vary the operating frequency of the array. In addition, selective manipulation of the dipoles allows the beam to be electrically steered.
Another approach to overcome the shortcomings of the GONZALEZ, et al. apparatus is disclosed in U.S. Pat. No. 5,864,322 for DYNAMIC PLASMA DRIVEN ANTENNA, issued to Gerald E. Pollon, et al. POLLON, et al. teach a dynamically reconfigurable reflective array consisting of a series of externally-controllable gas-filled cells or plasma structures. Each cell utilizes a series of horizontal and vertical electrodes to create reflective elements in ionized regions of the cells. By controlling the size, the geometry and the spacing of the ionized regions, a reconfigurable reflective array is created.
The inventive reconfigurable reflective array varies from the of POLLON, et al. in that a much simpler technology is utilized to implement the reconfigurable elements.
It is therefore an object of the invention to provide a low-cost, reconfigurable reflective array for dynamically simulating a non-planar reflector geometry.
It is a further object of the invention to provide a low-cost, reconfigurable reflective array for dynamically simulating a non-planar reflector geometry which may selectively operate across a wide range of frequency bands.
It is an additional object of the invention to provide a low-cost, reconfigurable reflective array for dynamically simulating a non-planar reflector geometry which may be electrically steered.
It is another object of the invention to provide a low-cost, reconfigurable reflective array for dynamically simulating a non-planar reflector geometry implemented using microelectromechanical systems switches (MEMS) for implementing variable length reflective elements.
It is a still further object of the invention to provide a low-cost, reconfigurable reflective array for dynamically simulating a non-planar reflector geometry wherein MEMS devices are controlled by a radiant energy supplied by optical fibers.
In accordance with the present invention there is provided a low-cost, frequency-tunable, steerable reflector array which is capable of simulating the electromagnetic effects of a parabolic or similarly-shaped reflector on a planar or conformal surface. A series of reflective scatterer sub-elements, connected serially to one another by switches, is used to form a reflective array disposed above a ground plane on a flat or curved surface. Activated reflective scatterers, arranged in configurations such as dipoles, crossed dipoles or the like are used to form the reflective surface. By properly controlling the length and spacing of the activated scatterer elements, it is possible to simulate the phase front of a reflecting surface such as a parabolic, spherical, cylindrical, or other similarly shaped reflector. In addition, the array may be electrically steered by selectively controlling the reflective scatterer configurations.